Third-order nonlinear optical effects in organic materials

Abstract

Abstract Whereas in normal linear materials light beams propagate without influencing each other, for large light intensities nonlinear effects can occur and create interactions between beams propagating through the same material. With such interactions light beams can be steered or switched by optical means only. The switching speed is as fast as the origin of the nonlinearity, which is in the case of electronic nonlinearities below femtoseconds (1015 s). Therefore large switching rates could potentially be reached interesting for applications in today's optical fiber networks. Third-order nonlinear optical properties are of prime interest because materials with large effects could enable all-optical switching, One of the material classes under consideration for large third-order nonlinearities besides glasses and semiconductors are organic molecules and polymers. In this work we mainly investigated three different molecular systems: polyenes, polytriacetylenes, and tetraethynylethenes. The development of relations between the molecular structure and the third-order nonlinear optical properties is a requisite to develop optimization guidelines on the path to potential applications. At first, calibration measurements of the third-order nonlinear optical properties of fused silica X~) have been performed to provide a reference value for the following third-harmonic generation measurements. With the two independent methods of cascading and gas phase third-harmonic generation the nonlinearities X>~)(-3(O, CD, 0), «i) =(2.0±0.2)-1O-22 m2/ y 2 at the wavelength 1064 nm and X~~\-3 W, W, (0, cu) =(1.6±0.2)·1O-n m2/ y 2 at 1907 nm have been detennined. The calibration approach with the gas phase third-harmonic generation has been demonstrated for the first time. We irnproved the sensitivity of the third-harrnonic generation measurement to an extend, that in the end 0.5 mg of a molecule was sufficient for a measurement of the nonlinearity. This improvement is very important since the synthesis of large quantities of novel materials can be very time consuming. In degenerate four wave mixing we introduced phase-sensitivity which enabled measurements not only 01' the magnitude but also 01' the phase of the nonlinearity and subsequent the determination of the real and imaginary part of the nonlinearity. With the white-light continuum spectroscopy we demonstrated a new method for the characterization 01' the material which simultaneously allows to measure the nonlinearity over a broad wavelength range. The main focus of this work is dedicated to three different molecular systems: polyenes, polytriacteylenes, and tetraethynylethenes. All of them have an extended conjugation 01' delocalized electrons and the possibility to substitute the endgroups 01' the conjugation by an electron donating 01' accepting functional group. In polyenes we obtained large nonlinearities which are among the largest ever reported. For asymmetrically electron donor-acceptor substituted polyenes with elongated conjugation second-order hyperpolarizabilities IIp to y =300·10-48 m5/ y 2 (21400'10-36 esu) are measured in the two-photon resonance. For all-optical applications, nonlinearities in the resonances enhancement are not interesting due to the loss mechanism bound to any resonance. We reach